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What is cancer?

Set of related diseases

Cancer is the name given to a set of related diseases. In all types of cancer, some of the body's cells begin to divide without stopping and spread to surrounding tissues.

Cancer can begin almost anywhere in the human body, which is made up of trillions of cells. Usually, human cells grow and divide to form new cells as the body needs them. When normal cells age or become damaged, they die, and new cells replace them.

However, in cancer, this orderly process is out of control. As cells become more and more abnormal, old or damaged cells survive when they should die, and new cells form when they are not needed. These additional cells can be divided without interruption and can form masses called tumors.

Many cancers form solid tumors, which are masses of tissue. Blood cancers, such as leukemias, usually do not form solid tumors.

Cancerous tumors are malignant, meaning that they can spread to nearby tissues or invade them. In addition, as these tumors grow, some cancerous cells may detach and move to distant locations of the body through the circulatory system or the lymphatic system and form new tumors away from the original tumor.

Unlike malignant tumors, benign tumors do not spread to nearby tissues and do not invade them. However, benign tumors can sometimes be quite large. When they are extirpated, they generally do not grow again, whereas malignant tumors do sometimes grow back. Unlike most benign tumors in other parts of the body, benign brain tumors can be life-threatening.

How cancer appears

Cancer is a genetic disease - that is, it is caused by changes in the genes that control the way our cells work, especially the way they grow and divide.

Genetic changes that cause cancer can be inherited from parents. They can also occur in a person's life as a result of errors that occur when dividing cells or DNA damage caused by some exposures of the environment. Environmental exposures that cause cancer are substances, such as chemical compounds in tobacco smoke and radiation, such as the sun's ultraviolet rays. (Our Cancer Causes and Risk Factors page has more information).

The cancer of each person has a unique combination of genetic changes. As the cancer continues to grow, additional changes will occur. Even within each tumor, different cells may have different genetic changes.

In general, cancer cells have more genetic changes, such as mutations in the DNA, than normal cells. Some of these changes may not be cancer related; Can be the result of cancer and not its cause.     

Cancer Causes

Genetic changes that contribute to cancer tend to affect three major types of genes - proto-oncogenes, tumor suppressor genes, and DNA repair genes. These changes are sometimes called "causers" of cancer.

Proto-oncogenes are devoted to normal cell growth and division. However, when these genes are altered in certain ways or are more active than normal, they can become cancer-causing genes (or oncogenes) by allowing cells to grow and survive when they should not.

Tumor suppressor genes are also dedicated to controlling cell growth and division. Cells with some alterations in tumor suppressor genes can be divided into an uncontrolled form.

DNA repair genes are dedicated to fixing damaged DNA. Cells with mutations in these genes tend to form additional mutations in other genes. Together, these mutations can cause the cells to become cancerous.

As scientists have learned more about the molecular changes that result in cancer, certain mutations have been found together in many types of cancer. Because of this, cancers are sometimes characterized by the types of genetic alterations that are believed to be causative, not only by the site of the body where they are formed and by the way cancer cells look under a microscope.     

Types of cancer


Carcinomas are the most common types of cancer. They are formed in the epithelial cells, which are the cells that cover the internal and external surfaces of the body. There are many types of epithelial cells, which often have a columnar shape when viewed under a microscope.
Carcinomas that begin in different types of epithelial cells have specific names:

Adenocarcinoma is a cancer that forms in epithelial cells that produce fluids or mucus. Tissues with this type of epithelial cells are sometimes called glandular tissues. Most breast, colon, and prostate cancers are adenocarcinomas.

Basal cell carcinoma is a cancer that begins in the lower or basal layer (at the base) of the epidermis, which is the outer layer of a person's skin.

Squamous cell carcinoma is a cancer that forms in squamous cells, which are epithelial cells that are below the outer surface of the skin. Squamous cells also cover many other organs, such as the stomach, intestines, lungs, bladder, and kidneys. Squamous cells look flat, like fish scales, when viewed under a microscope. Squamous cell carcinomas are sometimes called epidermoid carcinomas.

Transitional cell carcinoma is a cancer that forms in a type of epithelial tissue called transitional epithelium or urothelium. This tissue, which is composed of many layers of epithelial cells that can become larger or smaller, is found in the lining of the bladder, ureters and part of the kidneys (renal pelvis), and in some other organs . Some cancers of the bladder, ureters, and kidneys are transitional cell carcinomas.


Sarcomas are cancers that form in the bone and soft tissues, including muscles, adipose (fatty) tissue, blood vessels, lymphatic vessels, and fibrous tissue (such as tendons and ligaments).

Osteosarcoma is the most common bone cancer. The most common types of soft tissue sarcoma are leiomyosarcoma, Kaposi's sarcoma, malignant fibrous histiocytoma, liposarcoma, and protruding dermatofibrosarcoma.


Cancers that start in the tissues that make up the blood in the bone marrow are called leukemias. These cancers do not form solid tumors. Instead, a large number of abnormal white blood cells (leukemic cells and leukemic blasts) accumulate in the blood and bone marrow and displace the normal blood cells. The low concentration of normal cells in the blood can cause the body to carry oxygen to the tissues, that does not control the hemorrhage or that it does not fight infections.

There are four common types of leukemia, which are grouped according to how quickly the disease worsens (acute or chronic) and the type of globule where the cancer begins (lymphoblastic or myeloid).


Lymphoma is a cancer that begins in lymphocytes (T cells or B cells). These are white blood cells that fight diseases and are part of the immune system. In lymphoma, abnormal lymphocytes accumulate in lymph nodes and lymphatic vessels, as well as in other organs of the body.

There are two main types of lymphomas:

Hodgkin's Lymphoma - People who have this disease have abnormal lymphocytes called Reed-Sternberg cells. These cells are generally formed of B cells.

Non-Hodgkin's Lymphoma - This is a large group of cancers that begin on lymphocytes. Cancers can grow rapidly or slowly and can form B-cells or T-cells.

Multiple myeloma

Multiple myeloma is cancer that starts in the plasma cells, another type of immune cells. Abnormal plasma cells, called myeloma cells, accumulate in the bone marrow and form tumors in the bones of the whole body. Multiple myeloma is also called plasma cell myeloma and Kahler's disease.


Melanoma is cancer that begins in the cells that become melanocytes, which are cells specialized in producing melanin (the pigment that gives the color to the skin). Most melanomas form on the skin, but they can also form in other pigmented tissues, such as in the eyes.

Brain and spinal cord tumors

There are different types of brain tumors and spinal cord tumors. These tumors are called according to the type of cell where they were formed and where the tumor was first formed in the central nervous system. For example, an astrocytic tumor begins in star-shaped brain cells called astrocytes, which help keep nerve cells healthy. Brain tumors can be benign (noncancerous), or malignant (cancerous).     

Other types of tumor

Germ cell tumors

Germ cell tumors are a type of tumor that begins in the cells that make up sperm or eggs. These tumors can occur almost anywhere in the body and can be benign or malignant.

Neuroendocrine tumors

Neuroendocrine tumors are formed from cells that secrete hormones in the blood in response to a signal from the nervous system. These tumors, which can produce hormones in greater than normal amounts, can cause many different symptoms. Neuroendocrine tumors can be benign or malignant.

Carcinoid tumors

Carcinoid tumors are a type of neuroendocrine tumors. They are slow-growing tumors that are usually found in the gastrointestinal tract (most commonly in the rectum and small intestine). Carcinoid tumors can spread to the liver or other body sites, and can secrete substances such as serotonin or prostaglandins and cause carcinoid syndrome.     
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